The present invention relates generally to a mold for producing grid plates for lead batteries, especially grid plates for starter batteries, which tend to combine thickened frames and plate lugs with relatively thin grid ribs and webs.
Grid molds for starter battery grids are generally made from cast iron. Generally, two separate mold halves are held in a hinged mold holder or tensioning block such that each mold half is separately replaceable. To promote rapid cooling of the lead melt which is ultimately poured into the mold, the mold holder is provided with a series of cooling channels, which are preferably distributed in such a way that those surfaces which are most heated are exposed to the most effective cooling.
To obtain a good casting, pretreatment of the mold by coating it with talc, graphite or ground cork is absolutely necessary. Such mold-coating compounds, as a general rule, serve three purposes. A primary purpose of the coating is to provide insulation. However, because such compounds prevent the lead from adhering to the mold, such coatings also serve as a highly effective mold release or parting agent. Such coatings also permit air which is present in the mold cavity to escape to the nearest vent hole, at least over short distances and through pore spaces which are not filled by the lead.
However, it is just as important that the coating form a barrier to the transfer of heat from the lead melt to the mold material, which has relatively good thermal conductivity, to ensure that the molten lead does not prematurely solidify when it is poured into the mold, but rather is able to completely fill the mold cavity.
Consequently, renewal of the powder layer is required after each casting sequence. This operation may, for example, be advantageously carried out by means of a compressed air atomizer or the like. However, after a series of such applications, the dissipation of heat from the molten lead melt tends to take place much more slowly from those areas of the mold which form the lugs and frames than from those areas of the mold which form the latticework of the grid webs, where less material tends to accumulate. As a result, it has become common practice to scrape the coating layer which is present in the regions which form the lugs and frames of the grid plates, before the mold is again powdered, in order to equalize contact with the thermally conductive mold material. However, there are shortcomings in connection with such scraping procedures, primarily because the coating at the edges of the area being scraped inevitably becomes frayed, which subsequently causes relatively large pieces of the coating to chip off during use of the treated mold.
Equalization of the rate of cooling of the thick and thin parts of the plate grid can be achieved in a mold such as is known from DE-PS No. 1 155 833. Described in this patent is a mold having two one-piece mold halves, each of which has portions in the middle which contain recesses corresponding to the grid ribs and webs, as well as portions at the edges with a larger recess corresponding to the frame of the plate. The mid-portions of each of the mold halves are separated from the mold holders by an air gap. A direct heat-conducting connection is developed between the edge portions of the mold halves and the corresponding mold holders. As a result, the dissipation of heat from the thin latticework is delayed by the insulating air pockets which are formed.